As generally known, a TFT substrate is used for manufacturing an LCD.
FIGS. 33 and 34 show the structure of an example of the general TFT substrate, where gate wiring G, source wiring S, and the like are formed on substrate 86. In this structure, the gate wiring G and source wiring S are arranged in a matrix form on a transparent substrate 86 made of a glass material or the like. Each area surrounded by gate wiring G and source wiring S is a pixel (or picture element) portion 81, and TFT 83 is provided in each pixel portion 81.
The TFT 83 has a known etching-stopper type structure in which (i) gate insulating film 89 is provided on gate wiring G made of a conductive material such as aluminium (Al) or aluminium alloy (Al alloy), and on gate electrode 88 connected to the gate wiring G, (ii) semiconductor active film 90 formed by using amorphous silicon (a-Si) is placed over the gate insulating film 89 in a manner such that the semiconductor active film 90 and gate electrode 88 face each other via the insulating film 89, and (iii) drain electrode 91 and source electrode 92, each made of a conductive material such as Al or Al alloy, are formed on the semiconductor active film 90 opposite to each other.
In addition, ohmic contact films 90a, 90a are provided on either side of the semiconductor active film 90, where each ohmic contact film is made using amorphous silicon (a-Si) doped with high-density donor impurities such as phosphorus (P). Therefore, the etching stopper 93 is surrounded by drain electrode 91 and source electrode 92 provided on the ohmic contact films 90a, 90a, and by the semiconductor active film 90. Furthermore, transparent pixel electrode 95 made of indium tin oxide (abbreviated as “ITO”, hereinbelow) is provided to cover the upper surface of the drain electrode 91 and the area at the right side of the drain electrode.
In addition, passivation film 96 is provided over the gate insulating film 89, transparent pixel electrode 95, drain electrode 91, source electrode 92, and the like. On the passivation film 96, an oriented film (not shown) is formed, on which a liquid crystal is disposed, so that an active matrix LCD is provided. In the LCD, the orientation of the molecules of the liquid crystal can be controlled by applying an electric field to the molecules by using the transparent pixel electrode 95.
A method of manufacturing the TFT substrate as shown in FIGS. 33 and 34 will be explained. First, an Al or Al alloy thin-film layer is deposited on glass substrate 86 by using a general thin-film layer forming method such as a sputtering method, in which DC (direct current) power is applied to a target made of Al or Al alloy. In the next step, gate electrode 88 is formed by removing unnecessary Al or Al alloy portions outside the target Al or Al alloy portions necessary for forming the gate by using a photolithography method. In the following step, gate insulating film 89 made by using SiO2, SiNx, or the like, semiconductor active film 90, and etching stopper 93 are formed using a thin-film forming method such as the CVD method. In the next step, the ohmic contact film 90a, drain electrode 91, and source electrode 92 are formed on the portions formed in the previous step, by using the above-mentioned sputtering method and photolithography method. The formed drain electrode 91 and source electrode 92 are then masked, and a portion of the ohmic contact film 90a is removed so as to divide the ohmic contact film 90a into two portions. In the final step, passivation film 96 is formed using the CVD method or the like, thereby obtaining a TFT substrate.
Recently, the drive or control speed of the LCD has improved, and accordingly, a problem relating to delays of signal transmission due to resistance of some electrodes and wiring, such as the gate electrode, gate wiring, source wiring, and drain wiring, has become obvious. In order to solve the problem, using copper for forming electrodes and wiring, whose resistance is lower than that of Al or Al alloy, has been examined. Such a copper (Cu) wiring can be formed by a method similar to that used for forming a wiring made of Al or Al alloy, that is, a Cu layer is formed using a general sputtering method, and unnecessary portions of the Cu layer outside a target portion for forming the wiring are removed by using the photolithography method.
However, generally, Cu is easily affected by chemicals. Therefore, if Cu is used as a material for forming electrodes such as gate electrode 88 and wiring such as gate wiring G (abbreviated as “wiring material”, hereinbelow) in the LCD comprising a TFT substrate having the structure as shown in FIGS. 33 and 34, then when an etching material having the oxidizing capability (used for etch another layer in the latter process) is soaked into the Cu layer, the Cu layer may be etched and thus damaged. If the Cu layer is severely damaged, the Cu layer may be detached from the base substrate 86, or wiring may be broken or disconnected. Therefore, the possible etching material is limited.
In addition, if Cu is used as the wiring material, when the resist-detaching solvent used in the photolithography process soaks into the Cu film, the Cu film may be corroded by the solvent.
In the etching of the Cu film, the surface of the Cu film is oxidized. If an oxide layer of CuO, Cu2O, or the like is generated on the surface of the Cu film due to water or oxygen included in the air before the etching is performed, then the Cu film is etched and damaged even when an etching material having no oxidizing capability is used, and the wiring is broken and disconnected. Therefore, a Cu alloy may be used as a Cu wiring material for preventing the generation of such an oxide layer of CuO, Cu2O, or the like (on the surface of the Cu film). However, the wire resistance of the Cu alloy is larger than that of Cu; thus, a satisfactory effect obtained by using a material having a lower resistance is not expected in this case.
If gate electrode 88 is formed using a Cu film, Cu (atoms) may disperse into the gate insulating film 89, so that the withstand voltage may be degraded. In addition, if the substrate 86b is made of a glass material, Si (atoms) in the substrate 86 may be incorporated in the gate electrode 88, thereby increasing the resistance of the gate electrode 88.
If the drain electrode 91 and source electrode 92 are formed using Cu films, elements of electrodes 91 and 92, and semiconductor active film 90 disperse between each portion, so that the performance of the semiconductor active film 90 may be degraded.